1. Field Of The Invention
This invention relates to an improvement in oil film bearings of the type employed to rotatably support the journal surfaces of roll necks in a rolling mill.
2. Description of the Prior Art
In the typical rolling mill application of an oil film bearing, as depicted somewhat diagrammatically in FIGS. 1 and 2, the roll 10 has a neck section 12. The neck section 12 may be tapered, as shown, or it may be cylindrical. A sleeve 14 is received on the neck section 12 where it is rotatably fixed by means of one or more keys 13. The exterior of the sleeve defines the journal surface 16 of the roll neck. A bushing 18 has an internal bearing surface 20 surrounding and rotatably supporting the journal surface 16. The bushing is contained by and fixed within a chock 22. The chock is adapted to be supported in a roll housing (not shown), and is closed at the outboard end by an end plate 24 and cover 26. A seal assembly 28 is provided between the roll and the inboard end of the chock 22. The seal assembly functions to retain lubricating oil within the bearing while at the same time preventing contamination of the lubricating oil and inner bearing components by cooling water, mill scale, etc.
During normal operation of the mill, when the roll is rotating in the direction indicated by the arrow in FIG. 2 at speeds which are adequate for full hydrodynamic operation, a continuous flow of oil is fed through passageway 29 in the chock, feed openings 30 in the bushing and a rebore 32 in the bearing surface 20. From here, the oil enters between the bearing surface 20 and the rotating journal surface 16 to form a hydrodynamically-maintained oil film 34 at the bearing load zone "Z". The load zone is located on the side opposite to that of the load "L" being applied to the roll.
The oil ultimately escapes axially from between the journal and bearing surfaces 16,18 and is received in inboard and outboard sumps 36,38. From here, the oil is recirculated through filters, cooling devices, etc. (not shown) before being returned to the bearing.
If the rotational speed of the journal surface 16, the load L and the viscosity of the oil all remain within design limits, the bearing will continue to function satisfactorily, with an adequate oil film 34 being hydrodynamically maintained at the load zone Z. However, if one of these parameters should fall below its lower design limit, the hydrodynamically maintained oil film can deteriorate or collapse, causing metal to metal contact between the journal and bearing surfaces 16,20. If this should occur, the resulting friction will rapidly cause bearing failure.
Thus, from zero rotational speed at mill start up to the lower design limit for satisfactory hydrodynamic operation, the oil film 34 at the load zone Z must be created and maintained by means other than the hydrodynamic technique described above. To this end, and with reference additionallY to FIGS. 3-5, in the prior art conventional bearing assemblies, it is known to provide multiple hydrostatic recesses 40 in the bearing surface 20 at the load zone Z. The recesses 40 are interconnected by a network of passageways 42 to a positive displacement, constant volume high pressure oil pump 44.
As viewed radially from inside the bushing, the recesses 40 of the conventional prior art design are generally rectangular in configuration. The ends of the recesses are feathered as at 46, whereas the bottom 48 and side walls 50 are mutually perpendicular and thus define sharp bottom corners 52. The side walls 50 are perpendicular to the bearing surface 20 to thereby define sharp top edges 53.
With this type of arrangement, as the oil emerges from each recess 40 to hydrostatically form the oil film 34, it encounters very high resistance and thus experiences a significant pressure drop as it is forced in the axial direction between the sharp edges 53 and the bearing surface 20. The net result is that in order to maintain a given oil pressure in the film 34, a substantially higher oil pressure must be maintained in the recess 40. This in turn means that the pump 44 must work harder, and the entire lubrication system must be designed to operate at higher pressures.
It will also be seen that the oil emerging circumferentially from each recess 40 at its feathered ends 46 encountered significantly less resistance as compared to that encountered by the oil emerging axially past the sharp corners 53. This encourages circumferential flow at the expense of axial flow, which in turn adversely affects oil pressure field distribution throughout the load zone. The oil pressure field supports the load at the load zone.
Other disadvantages of the conventional design include high stress concentrations at the bottom corners 52, which can create cracks and cause bearing failure. Also, the sudden change in flow area at the sharp edges 53 results in relatively high fluid velocities, which in turn hasten metal erosion.
The objective of the present invention is to provide a bushing having novel and improved hydrostatic recess configurations which either avoid or at the very least, substantially minimize the problems associated with the prior art.